The invention relates to strands used in civil engineering structures or other structures.
Individually protected strands comprising multiple twisted metal wires (also referred to as threads) are known; commonly there are six steel wires forming a helix around a central steel wire. The metal wires frequently have undergone electrochemical treatment (galvanization, galvanization, etc.). They are also enclosed in an external sheath, generally of plastic. The space between the twisted metal wires and the sheath is filled with protective material.
These individually protected strands are conventionally used to create bridge stays, and have been found to be effective in protecting these stays against corrosion.
The protective material used in these individually protected strands generally consists of wax, for example petroleum wax, or grease. These individually protected strands therefore cannot efficiently transmit significant axial (i.e. tangential) stresses from their external sheath to their twisted metal wires.
This is why such individually protected strands cannot be used in applications where axial forces, likely to cause the sheath to slide on the metal wires, must be applied to the strands. Such is the case for suspension bridges or other suspended structures, as well as bridges with saddles. In a suspension bridge for example, main cables comprising bundles of individually protected strands must withstand frictional stresses exerted parallel to their axis, and these stresses are for example transmitted by clamping collars from which the deck of the bridge is suspended by means of hangers.
One solution is to remove the sheath from the strands and use direct attachment to the metal wires. However, this lessens the corrosion protection and therefore the durability of the structure of which the strands are a part.
“Cohesive” strands are also known. Compared to the sheathed-greased strands or sheathed-waxed strands mentioned above, the filling product used in cohesive strands, between the twisted metal wires and the sheath, is a protective material that adheres to the metal wires and to the inner surface of the sheath, typically an adhering polymer.
Cohesive strands are therefore particularly useful when it is necessary to transfer axial (i.e. tangential) stresses from the sheath to the metal wires, for example in the main cables of suspension bridges, in bridges with saddles, or other applications.
Details on cohesive strands can be found in EP 0 855 471 for example.
One disadvantage of cohesive strands lies in their manufacturing cost, while their adhesion properties are potentially only used within a very small portion of their length: the cumulative lengths of the collars in the case of a suspension bridge, the length of the saddle in the case of a bridge with saddles, etc., which can represent less than 10% of the total cable length.
In addition, it is technically complex to obtain satisfactory cohesiveness on an interface of protective material/plastic material (e.g. HDPE). This complexity is further increased when the interface in question is extensive. Such cohesiveness for the entire length of a strand can be difficult to obtain other than in the factory during manufacture of the strand and under carefully controlled conditions.
An object of the invention is to propose a strand which reduces at least some of the above disadvantages.